This invention relates to a blowing agent composition and a process for producing thermoplastic resin foams using such a composition.
In a known process for obtaining foams by heating a thermoplastic resin preform containing a blowing agent, azodicarbonamide (hereinafter referred to as "AC"), the internal temperature of the preform is raised up by decomposition heat of the blowing agent during foaming and hence such internal temperature usually remains higher than the temperature on the external surface. Therefore, although AC in the inside of the foamed sheet is perfectly decomposed and whitened, AC in the surface layer undergoes insufficient decomposition and keeps its yellow color. Thus, foams obtained by using AC as blowing agent are almost unexceptionally colored in light yellow even when no specific coloring is made with a pigment or such, and it is hardly possible to obtain perfectly white products. Naturally, colored (yellow) products are of poor commercial value.
Several methods have been proposed for preventing such yellowing of foams obtained by using AC as blowing agent. There are known, for example, the following methods: a thermoplastic resin film is laminated on the surface of an expandable polyolefin resin preform so that the preform surface will be treated as an internal layer when foamed under heating, thereby to increase the extent of foaming in the surface layer and to allow obtainment of the perfectly whitened foams free of any undecomposed AC (Japanese Pat Appln Kokai (Laid-Open) No. 36773/74); ultraviolet rays are applied to the surface of a foam (Japanese Pat Appln Kokoku (Post-Exam Publn) No. 7473/68); a colored foam is treated with a heated hydrazine solution (Japanese Pat Appln Kokoku (Post-Exam Publn) No. 18636/71). Nevertheless, none of these methods is capable of attaining a well satisfactory result.
U.S. Pat. No. 3,278,466 shows techniques for foaming rubber or thermoplastic resins by using a blowing agent composition comprising AC and a second component such as benzene sulphonhydrazide, dinitrosopentamethylenetetramine or the like. However, use of such blowing agent composition results in an unstabilized foamed condition, and it is very difficult to obtain highly expanded foams with excellent appearance and smooth surface.